At present there are many technical and industrial situations in which it is necessary to avoid or minimize the effects of static electricity. Normally this is itself innocuous, but the effects of its discharge can be both catastrophic and expensive. As a fairly obvious example, fires and explosions may be caused by sparking discharge of static electricity in industrial plants. However at present there is a great requirement for antistatic materials in the electronics industry, where each year many millions of pounds worth of sophisticated components are either destroyed or so wounded as to impair their performance, reliability or life time, by the discharge of static electricity through sensitive parts thereof. Although the static charges that accumulate at e.g. the surfaces of the insulating boxes in which such components are carried around are usually small, they may have a very high potential and can therefore cause damagingly high currents to flow, albeit for very short periods.
At present there are two primary known regimes for avoiding the effects of this static discharge at surfaces. The first involves the prevention of static build-up, and uses conductive materials, i.e. materials with surface resistivities of less than about 10.sup.5 ohms per square. There is however no guarantee that e.g. an electronics device can be completely shielded from static build up during manufacture, shipping etc. and in this regime the discharge of any such build-up is liable to be damagingly rapid.
A second regime is termed "antistatic" and is intended to allow discharge of any static build-up. At present this is generally achieved by the use of manufacturing aids moulded from polymers loaded with a conducting filler, usually a finely divided carbon black. With these loaded polymers surface resistivities up to about 10.sup.6 ohms per square may be achieved. Unfortunately for many applications such an antistatic material still has insufficient resistivity to prevent potentially damaging rates of electrostatic discharge, but it has in practice not been possible to provide a loaded polymer with the desired greater resistivity. The reason for this is as follows. The role of the filler in a conducting or antistatic loaded polymer is to provide electrically conducting pathways through the insulative polymer base. Conduction through the material occurs by electron tunnelling from one conductive particle to another, and the degree of tunnelling that can occur depends on the mean free paths between these particles. Above a certain critical distance, no tunnelling can take place. In order to obtain a loaded polymer of the highest possible resistivity, it is of course necessary to reduce the proportion of conducting filler as far as possible. However, assuming that the filler particles are all the same shape and homogeneously mixed, a concentration will be reached at which all the particles are at the critical distance apart. Any further reduction in the amount of filler below this critical concentration will result in no conduction by tunnelling and the composition will suddenly become an insulator; the desired intermediate resistivity cannot be achieved. In an attempt to achieve these resistivities, conductive filler particles of very high aspect ratio have been used such as e.g. chopped carbon fibers and high conductivity long chain carbon blacks. These are normally very expensive and in any case difficult to use as loading materials because they tend to be non-wettable and have a strong tendency to aggregate. Even using these filler materials it has not been found possible to produce adequately resistive compositions; because of the critical concentration problem discussed above they are either substantially insulating or excessively conductive. A further problem with carbon black loaded polymers is a tendency to change in resistivity when subjected to strain, especially when vulnerable high aspect ratio carbon chains are involved.
It has also been sought to avoid the problem of electrostatic build up by using static dispersive coatings, which render the surface of a material hydroscopic and derive their ability to disperse static charge from the adsorption of ambient moisture. If anything these tend to be rather strongly insulating, with surface resistivities greater than 10.sup.8 ohms per square. Moreover they are easily damaged, short lived and, relying as they do on ambient humidity, are variable in their effectiveness.
Clearly it would be desirable to provide a resistive composition which can include the resistivity range between 10.sup.6 and 10.sup.8 ohms per square, while also being cheap and convenient to manufacture. It would be particularly desirable to be able to formulate with this convenience a composition with a chosen surface resistivity anywhere in the large range of, say, 10.sup.2 to 10.sup.11 ohms per square.